The distribution, or streaming, of program audio and video in analog, digital, or compressed data over satellite and other terrestrial distribution paths can cause a timing or delay skew of the audio signal relative to the video signal creating the well known “lip-sync” problem. The causes are varied but generally occur when the audio is processed separately from the video with different processing or distribution delays. For example, the video may be sent over a satellite path and the audio over a lower-delay terrestrial path creating the need to re-sync the audio and video at some common destination point. Also separate coding and decoding (CODEC) delays of compressed multi-channel audio along with video creates the need to re-sync the audio relative to the video.
In many cases the multi-channel program audio associated with a program video signal has the correct timing relationship at some origination point and it is desired to re-create that relationship by delaying the audio or video at the destination point to correct for the different propagation or CODEC delays. This can be done “out-of-service” by inserting an audio tone burst or equivalent for the program audio at the origination point that is synchronous with a video flash, rotating wheel, electronic clap-board, or equivalent such that the AV delay can be measured and corrected at the destination point.
However, this approach has two major disadvantages. Firstly, it requires taking the normal program audio and video out-of-service for the test. Secondly, the AV processing delay may change over time due to CODEC variation and routing changes making the out-of-service correction value measured no longer valid.
U.S. Pat. No. 6,246,439 titled “Transparent embedment of data in a video signal” (“the '439 patent”) describes a different approach in which an invisible watermark or some other metadata path is added to the video signal in order to send an audio envelope “signature curve” to the decoder for measurement and correction. In this manner, audio and video can be continuously adjusted into synchronization. This approach was used in the AVDC100 Audio-to-Video Delay Corrector (now discontinued) available from Tektronix, Inc. of Beaverton, Oreg. However, watermarking of the video signal is sometimes not acceptable and the watermark may not be detectable after video effects and scaling.
Alternatively, a video signature curve could be embedded into the audio channel as metadata or via an audio watermark in some form in order to measure and correct the AV delay in the same way as the '439 patent. However, the audio signal watermark modification may not be acceptable and the metadata path is often not preserved across the distribution chain.
What is desired is a method of measuring and correcting AV delay that overcomes the deficiencies of these previous approaches.